1. Field of the Invention
The present invention relates to an improved method for monitoring the durability i.e. service life of components of thermal power plants where equivalent hours of operation, in functional relation to thermal as well as mechanical influences, are counted. Equivalent hours of operation can be defined as the operating period which causes, relative to design characteristics, the same fatigue of material as the operating period actually performed, at the stresses which actually did arise during this period. The invention is further concerned with a novel apparatus by means of which the novel method can be performed.
Components of power plants to be monitored are primarily those parts of thermal machines and equipment which are subjected to fluctuating temperatures as well as high and varying stresses while they are operating.
Highly stressed structural members of thermal machines and equipment, for example of steam turbines, gas turbines and steam boilers, as well as components of power plants, such as pipes, manifold collectors and armatures, are designed for a specific operating period, based on the occurring stresses which are continuous. Assuming an ideal, stable operation at constant stress or load and at constant temperature, the continuous stress can be correlated with a planned service life which can be selected from the service life graph of the material being used.
Higher stresses and higher temperatures will reduce the service life correspondingly. When loads are changing, especially during starts and shut-downs of the thermal power plants, additional stresses are created by the one-sided heating or cooling of the components, thereby reducing their service life. However, in the case of installations which are operated under primarily stable conditions, as in the case of base-load power plants, it will be possible to make proper provisions for transitory higher stresses by addition of a safety factor when designing the lay-out, if the transient peak loads are kept within specific, pre-determined limits. It is for this purpose that devices heretofore proposed, for example in German patent applications published, DAS 1,401,453 and DAS 1,698,476 serve to detect and limit the transient peak load, namely by measuring continuously the load and comparing it with a temperature-related, maximum allowable load.
In the case of medium load installations, and especially in the case of peak load power plants which are started and shut down daily, the layout design on the basis of stable operating characteristics and with wide margins for safety becomes uneconomical and unreliable, because it is difficult to predict the life of the components which are subjected to the greatest stresses under changing operating conditions.
Devices have been developed for power plants (jet engines), as disclosed for example in U.S. Pat. No. 3,237,448, to measure and register the service life of a component subjected to high temperatures by counting equivalent hours of operation. However, power plants for aircraft differ from stationary power plants because of their thin-walled constructions which are necessary in order to attain a necessarily desirable low ratio of weight to power. Therefore, rotors and housing will heat up quickly and additional thermal stresses will not be generated for longer periods of time as is the case of the solidly constructed stable, stationary power plants.
Also known is a method to determine the permissible total hours of operation of thermal machines in functional relation to operating conditions, as disclosed for example in Swiss Pat. No. 416,190, where hours of operation are counted and multiplied continuously by a factor which is a function of the prevailing plant conditions. This factor can be a function of a temperature. The factor can also be a function of several parameters, for example a function of temperature combined with that of the rotary speed of a machine; however this Swiss patent is silent concerning the manner of detecting and processing such additional factors.
Finally, there is known an electrical measuring circuit for monitoring the service life of steam boilers and high-pressure steam pipes, using a device at the component to be monitored which measures the temperature of the steam, reference being made to German published Pat. application DAS 1,243,208, wherein one electrical quantity, representing a first measured quantity, steam temperature or steam pressure, energizes an impedance network with at least one resistor, its resistance decreasing, in a substantially exponential manner, with the electrical quantity, and where the output of the network activates an electrical counter, whereby at least one resistor of the network is varied in accordance with a second measured quantity, steam pressure or steam temperature.
2. Description of the Invention
The present invention is based primarily on the state of the art as disclosed by the above-mentioned Swiss Pat. No. 416,190. The principal objective of the invention is to establish an improved method for monitoring the service life of components of thermal power plants, especially to make feasible also a precise detection of the intermittent thermal stresses, generated in metals during the heating-up or cooling-off process, stresses which will occur primarily in connection with changes in load, or starting conditions.
The improved method, and the apparatus to be used for its practical application, should make it possible to take into account, in addition to the thermal stresses mentioned above, other influences which may occur under various operating conditions and which might affect the service life of components, such as purely mechanical stresses which will occur, for example due to centrifugal forces or internal pressure. Obviously, the influence of the temperature level at which the machine, or component to be monitored, is being operated, should always be taken into consideration when the service life is being ascertained. Thus, it is a primary objective of the invention to provide a novel method, and apparatus which will improve the monitoring systems heretofore proposed, thereby making possible a more reliable determination, or monitoring respectively, of the service life of components in thermal power plants.